In general, a cold chamber-type die casting machine comprises as an injection means a horizontal cylinder having an inlet opening in an upper side wall thereof near its end, and a charge of molten metal is introduced into the cylinder through the inlet opening and injected into a die cavity communicating with the cylinder by means of a piston slidably moving in the cylinder. Since the molten metal directly drops onto and collides with the inner surface of the cylinder particularly just below the inlet opening, it is likely that the inner surface of the cylinder is subjected to erosion in such a portion, and that it is worn by sliding contact with the piston. When the inner surface of the cylinder is damaged by these erosion and wear cycles, the molten metal can leak into a gap between the cylinder and the piston, resulting in an increase in the sliding resistance of the piston. This in turn reduces the injection rate, which leads not only to the deterioration of qualities of injection products but also to the reduction of productivity and to a further increase in damage of the inner surface of the cylinder. If a large amount of a lubricating agent is used to reduce the sliding resistance between the cylinder and the piston and to prevent seizure, it could be entrained into the molten metal as an impurity, resulting in the deterioration of product quality.
In view of these problems, cylinders made of nitrided steel having a hardness of H.sub.R C 65 (H.sub.V 832) or so have conventionally been used. The use of such nitrided steel decreases the wear and erosion of the inner surface of the cylinder, but nitrided steel is still unsatisfactory in terms of the durability of the cylinder and the quality of products cast by using such a cylinder.
Accordingly, various proposals have been made to provide a die casting cylinder in which a heat-resistant, wear-resistant ceramic cylinder is fixed in a steel cylinder, into which a piston is inserted.
U.S. Pat. No. 3,664,411 discloses a die casting cylinder comprising a metal casing and a ceramic liner having good resistance to corrosion by molten metals and thermal fatigue resistance, the ceramic liner affording an external surface of slightly tapered form and the casing affording an internal surface having a taper complementary to that of the liner, whereby the liner and the casing are assembled together as a press fit so that the liner is supported by the casing at all positions along its length.
Japanese Patent Laid-Open No. 53-70034 discloses a die casting apparatus comprising a melt injection cylinder in which a plurality of separate ceramic sleeve pieces are assembled.
Japanese Patent Laid-Open No. 61-67555 discloses an injection sleeve for die casting, having a dual structure consisting of an inner cylinder and an outer cylinder, between which a water-cooling jacket is provided.
Further, Japanese Patent Laid-Open No. 61-103658 discloses an injection cylinder for a die casting machine comprising a cylinder body into which a piston is inserted, and an inner cylinder made of a nonferrous metal material having extremely high heat resistance and wear resistance such as a ceramic or a cermet, which is fit under proper pressure in the cylinder body at least near an inlet opening for a molten metal, the cylinder body being provided with at least one path for introducing a heating medium.
The above-mentioned structures in which ceramic inner cylinders are fit in the cylinder bodies are extremely effective for improving the wear resistance and corrosion resistance of the cylinders, but there is still a room for improvement in that the temperature of the molten metal introduced into the cylinders is lowered. In a die casting method, since casting is generally carried out under the conditions that the temperature of a molten metal is as close to its solidification temperature as possible, it is required that the cylinder has high heat insulation, namely, high temperature-keeping property in addition to the above properties to prevent the temperature decrease of the molten metal. In this respect, the cylinder having a ceramic inner cylinder fixed thereto has slightly improved heat resistance and temperature-retaining properties as compared to conventional steel cylinders, but it still fails to satisfy the requirements in the die casting method.
In addition, when the molten metal is introduced into the cylinder, a lower portion of the cylinder, which is brought into contact with the molten metal, is preferentially heated. Accordingly, because of the difference in thermal expansion between the lower and upper portions of the cylinder, the cylinder tends to be warped upward. As a result, cracking and breakage are likely to take place in the ceramic inner cylinder.